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Thread: Replacing nest with evohome

  1. #1
    Automated Home Lurker
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    Default Replacing nest with evohome

    I have a nest install off a condensing boiler today - so I have the gas boiler and a hot water tank. My heat link is upstairs with the hot water tank.

    I am looking at replacing nest with evohome but wonder how complex the install will be - is it as simple as rewiring the nest heatlink box to the evohome kit?

    I believe I will need the evohome WiFi controller set and also the hot water kit plus the TRVs I would need is that correct? Do I need to use a relay for both the heating and for the hot water or is it simpler than that?

    If anyone can point to a simple guide that would be great, I am fine with setting up the system and the TRVs etc itís just getting the wiring right I would like to ensure I am ok with it calling in someone if necessary

  2. #2
    Automated Home Guru
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    You need to determine which is the closest format of your heating, for example mine is a Y-Plan (so has a 3-port valve) and I use a BDR91 for the heating demand and a BDR91 for the HW demand, with the Wi-Fi tank thermostat.

    The BDR91 contacts can usually be wired easily into an existing system as it provides a pair of volt-free contacts like a simple thermostat would, the contacts close when there's heating demand. For other systems like the so-called s-plan you still need two BDR91's one for the heating demand valve and one for the HW demand valve.

    There are some really good wiring diagrams produced by Honeywell.

    I used the Honeywell Wiring junction box https://www.screwfix.com/p/honeywell...tion-box/66443 which makes so easy and the wiring can be replicated exactly as per the internal connectors, makes errors very difficult to induce. In some respects the Drayton Wiring centre is better as there are more contacts, which saves having to insert two or more wires into one terminal, the Honeywell variant is the same, but the £8 or so is well worth the investment.

    Once installed the system works really well and I'm getting at least 25% saving over the Nest plus the level of comfort is vastly superior whilst using less gas, once the major benefits or us was a room that was always cold with a basic bang-bang control system, is the zone can demand its own heat, its wonderful.

    Bosch are now doing their own variant, their radiator controllers are nice and quiet a factor to consider for the Evohome as my wife still can't come to terms with the whirring controllers on each radiator. For example we have two radiators in out bedroom and at 0700 they both make quite a sound as they switch on!

  3. #3
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    Thanks for that - the bosch system looks nice enough, but from what I can see it cant call the boiler to heat, so would only work if the boiler was firing, on a timer or if some other thermostat called it to fire up - or have I missed something there?

    will have a look at my system and the wiring into the nest as it stands... Ideally I figure out the equivilant connectors and replace that - was just thrown by the seperate hot water kit thats all.

  4. #4
    Automated Home Guru
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    You need the Hub, that calls for heat. In its simplest form the Nest has two no volt contacts that open/close to demand heat and the Evohome can do that easily via a BDR91, youíd have no control over HW but you can leave the current programmer and tank thermostat as is to provide HW. If any Evohome zone demands heat you want the boiler service that demand and that takes just two wires. You could replace the Nest controller with a BDR92 and thatís it.

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